Cannabis sativa has been used for the treatment of pain for many years. Δ9-tetrahydrocannabinol is a major active ingredient from Cannabis sativa and an agonist of cannabinoid receptors (Pertwee, Brit J Pharmacol, 2008, 153, 199-215). Two cannabinoid G protein-coupled receptors have been cloned, cannabinoid receptor type 1 (CB1 Matsuda et al., Nature, 1990, 346, 561-4) and cannabinoid receptor type 2 (CB2 Munro et al., Nature, 1993, 365, 61-5). CB1 is expressed centrally in brain areas, such as the hypothalamus and nucleus accumbens as well as peripherally in the liver, gastrointestinal tract, pancreas, adipose tissue, and skeletal muscle (Di Marzo et al., Curr Opin Lipidol, 2007, 18, 129-140). CB2 is predominantly expressed in immune cells, such as monocytes (Pacher et al., Amer J Physiol, 2008, 294, H1133-H1134), and under certain conditions, also in the brain (Benito et al., Brit J Pharmacol, 2008, 153, 277-285) and in skeletal (Cavuoto et al., Biochem Biophys Res Commun, 2007, 364, 105-110) and cardiac (Hajrasouliha et al., Eur J Pharmacol, 2008, 579, 246-252) muscle. An abundance of pharmacological, anatomical and electrophysiological data, using synthetic agonists, indicate that increased cannabinoid signaling through CB1/CB2 promotes analgesia in tests of acute nociception and suppresses hyperalgesia in models of chronic neuropathic and inflammatory pain (Cravatt et al., J Neurobiol, 2004, 61, 149-60; Guindon et al., Brit J Pharmacol, 2008, 153, 319-334).
Efficacy of synthetic cannabinoid receptor agonists is well documented. Moreover, studies using cannabinoid receptor antagonists and knockout mice have also implicated the endocannabinoid system as an important modulator of nociception. Anandamide (AEA) (Devane et al., Science, 1992, 258, 1946-9) and 2-arachidinoylglycerol (2-AG) (Mechoulam et al., Biochem Pharmacol, 1995, 50, 83-90; Sugiura et al., Biochem Biophys Res Commun, 1995, 215, 89-97) are two major endocannabinoids. AEA is hydrolyzed by fatty acid amide hydrolase (FAAH) and 2-AG is hydrolyzed by monoacylglycerol lipase (MGL) (Piomelli, Nat Rev Neurosci, 2003, 4, 873-884). Genetic ablation of FAAH elevates endogenous AEA and results in a CB1-dependent analgesia in models of acute and inflammatory pain (Lichtman et al., Pain, 2004, 109, 319-27), suggesting that the endocannabinoid system functions naturally to inhibit pain (Cravatt et al., J Neurobiol, 2004, 61, 149-60). Unlike the constitutive increase in endocannabinoid levels using FAAH knockout mice, use of specific FAAH inhibitors transiently elevates AEA levels and results in antinociception in vivo (Kathuria et al., Nat Med, 2003, 9, 76-81). Further evidence for an endocannabinoid-mediated antinociceptive tone is demonstrated by the formation of AEA in the periaqueductal grey following noxious stimulation in the periphery (Walker et al., Proc Natl Acad Sci USA, 1999, 96, 12198-203) and, conversely, by the induction of hyperalgesia following antisense RNA-mediated inhibition of CB1 in the spinal cord (Dogrul et al., Pain, 2002, 100, 203-9).
With respect to 2-AG, intravenous delivery of 2-AG produces analgesia in the tail flick (Mechoulam et al., Biochem Pharmacol, 1995, 50, 83-90) and hot plate (Lichtman et al., J Pharmacol Exp Ther, 2002, 302, 73-9) assays. In contrast, it was demonstrated that 2-AG given alone is not analgesic in the hot plate assay, but when combined with other 2-monoacylglycerols (i.e., 2-linoleoyl glycerol and 2-palmitoyl glycerol), significant analgesia is attained, a phenomenon termed the “entourage effect” (Ben-Shabat et al., Eur J Pharmacol, 1998, 353, 23-31). These “entourage” 2-monoacylglycerols are endogenous lipids that are co-released with 2-AG and potentiate endocannabinoid signaling, in part, by inhibiting 2-AG breakdown, most likely by competition for the active site on MGL. This suggests that synthetic MGL inhibitors will have a similar effect. Indeed, URB602, a relatively weak synthetic MGL inhibitor, showed an antinociceptive effect in a murine model of acute inflammation (Comelli et al., Brit J Pharmacol, 2007, 152, 787-794).
Although the use of synthetic cannabinoid agonists has conclusively demonstrated that increased cannabinoid signaling produces analgesic and anti-inflammatory effects, it has been difficult to separate these beneficial effects from the unwanted side effects of these compounds. An alternative approach is to enhance the signaling of the endocannabinoid system by elevating the level of 2-AG, the endocannabinoid of highest abundance in the central nervous system (CNS) and gastrointestinal tract, which may be achieved by inhibition of MGL. Therefore, MGL inhibitors are potentially useful for the treatment of pain, inflammation, and CNS disorders (Di Marzo et al., Curr Pharm Des, 2000, 6, 1361-80; Jhaveri et al., Brit J Pharmacol, 2007, 152, 624-632; McCarberg Bill et al., Amer J Ther, 2007, 14, 475-83), as well as glaucoma and disease states arising from elevated intraocular pressure (Njie, Ya Fatou; He, Fang; Qiao, Zhuanhong; Song, Zhao-Hui, Exp. Eye Res., 2008, 87(2):106-14).